The present invention relates to a grid construction for a fluidized bed reactor and a method of using such a grid construction for removing coarse material from the bottom of a fluidized bed reactor. More particularly, the present invention relates to a durable grid construction optimized for reliable and efficient removal of coarse material from a fluidized bed reactor.
Fluidized bed reactors comprise a reaction chamber, defined by vertical walls, having a fluidized bed of solid particles maintained therein, a windbox in a lower portion of the reactor under the reaction chamber, and a grid construction between the reaction chamber and the windbox for supporting the fluidized bed in the reaction chamber. In the grid construction, there are typically means, such as nozzles, for distributing fluidizing air or other gas into the reaction chamber from the windbox.
Fluidized bed reactors are used for different processes, such as combustion or gasification of solid fuel, which may produce coarse residue material which accumulates on the bottom of the reaction chamber, i.e., on the grid. The accumulation of the coarse material may be a problem especially when solid fuels rich in non-combustible material, such as stones or scrap iron, are processed in the reaction chamber, or when the process generates such coarse material, as, for example, ash agglomerations. Coarse material accumulating on the grid should be removed before it interacts detrimentally with the operation of the fluidized bed, e.g., by preventing appropriate distribution of fluidizing gas or process gas into the reaction chamber. Thus, effective equipment is needed in fluidized bed reactors for adequate removal of coarse material from the bottom of the reactor. Usually at least one discharge duct is provided at the bottom of the reactor chamber for withdrawing solids (i.e., coarse material) from the reactor.
A conventional material discharge channel or a few material outlet pipes placed at the bottom of a reactor chamber are, however, often not sufficient for efficient removal of coarse material, as random movement generated by the fluidizing air is not capable of transporting heavy or unfavorably-shaped pieces horizontally for long distances. Also, the migration of material along the bottom of the reactor chamber is especially hampered when the bottom has upwardly projecting air nozzles or other irregularities, to which the coarse material may stick.
An attempt to improve the removal of material from the bottom of a fluidized bed is to use directional fluidizing air nozzles which, by blowing, guide the material towards the discharge opening or duct, as disclosed, e.g., in U.S. Pat. No. 5,395,596. However, separate air nozzles are prone to erosion, and coarse material may stick into the nozzles.
Another approach is that the bottom of the reaction chamber could be stepped or slanted downwards towards the outlet, whereby the gravitation assists the horizontal migration of the material. Such a solution has been presented, for example, in U.S. Pat. No. 4,372,228. The problem with these solutions may, however, be abrasion of the nozzles and regions in their vicinity, caused by the moving material and by the transport air being fed at a high velocity. It is also difficult to ensure an even air feed over the entire grid area due to the different bed pressures at various nozzles, at least when the bottom has significant level differences.
U.S. Pat. No. 4,748,916 discloses a nozzle with a substantially horizontal upper portion, and an arrangement of such nozzles disposed at the bottom of a fluidized bed combustor. The nozzles direct two diverging air jets from their front ends horizontally towards areas between adjacent nozzles. This patent aims to provide a durable nozzle which supplies air in such a manner that coarse material can be moved along the bottom of the bed. However, the construction provides a highly varying flow velocity field at the bottom, and thus, there is a risk of erosion or accumulation of coarse particles in low velocity areas.
As can be realized from the above description of the prior art, there is a need for a new grid construction by which coarse ash and other non-combustibles can be efficiently and reliably transported to ash withdrawal from the entire bottom area of a fluidized bed reactor.